1. Field of the Invention
This invention relates to bearings and more particularly to flywheel power systems for satellites and other applications, and, in particular, to a secondary bearing for a flywheel IPACS (Integrated Power and Attitude Control System) that enables better impact protection and recovery from power loss.
2. Description of the Related Art
IPACS is a system that performs attitude control and energy storage for commercial and military satellite applications that have one or more energy storage flywheels. For example, energy from a satellite's solar panel is stored in the IPACS flywheel system as kinetic energy during the sunlit portion of the orbit. The flywheel has an integral motor/generator that is accelerated to very high rotational speeds while such external energy is available from the satellite's solar panels. The kinetic energy stored in the flywheel enables the system to generate electrical energy when the power demand of the satellite is greater than the output of the solar array. In addition, the stored momentum of the system can be used to provide attitude control for the satellite.
Since a flywheel may rotate at tens of thousands of RPM's, the bearing system for the flywheel is crucial. The flywheel primary bearing may be a non-contact magnetic bearing. A secondary mechanical bearing (SMB) may be included in the system to provide backup, if electrical power is lost while the flywheel is rotating. The SMB prevents the magnetic bearing components, namely the rotor and stator, from contacting each other during such a power loss or overload and also enables rotation and power generation by the flywheel during such conditions. The surfaces of the primary bearing and the contacting surfaces of the SMB are in close proximity during normal operation of the magnetic bearing. This close proximity is to ensure that the primary bearing will move little, if at all, during a power failure to prevent primary bearing contact.
However, after a primary bearing failure, the rotor and mating face of the SMB may slide relative to each other for a short period of time during initial contact which can occur under high load and velocity. Under these conditions wear can occur. Since the clearance between magnetic bearing rotor and stator is very small, the reduction by wear of the contact face thickness of the SMB can result in magnetic bearing surface contact and damage if the SMB is engaged. In addition, impact damage of the SMB contact faces can result if the primary bearing suddenly loses power and drops to the SMB. If the magnetic bearing surfaces are damaged, the primary bearing may not be usable and the satellite may not be able to store energy or provide power at peak demand times. Also, the attitude control function could be reduced or eliminated, possibly dramatically affecting the usefulness and useful life of the satellite.
Conventional coatings for SMB use may be hard and fragile and can spall when subjected to impact. Such characteristics may be aggravated by the high temperatures which occur and tend to soften such coating materials as carburized or nitrided steel, tungsten carbide, and chromium carbide. Maintenance is difficult if not impossible in space, making reliable and robust SMBs highly desirable. Thus, there is a need for a flywheel system that has an SMB that is more robust and that addresses one or more of the drawbacks identified above.